Heart rate monitors (HRM) are now considered an important part of one's workout gear--some exercisers would no sooner work out without an HRM than they would without their shoes. Some consider an HRM even more important than an iPod!
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This diary will look at the one HRM feature that has driven this increase in popularity--estimating calories burned during a workout. I will attempt to describe the scientific rationale behind this feature and point out the benefits and shortcomings. I will focus primarily on products from Polar, not to single them out in any way positively or negatively, but because they have the largest market share and their products are the ones that most exercisers--and myself--are familiar with.
From humble beginnings as a simple device to count heartbeats, HRMs have now evolved into virtual coaches and trainers--tracking calories, measuring fitness levels, suggesting workouts and even measuring state of recovery from your previous workout.
Many heart rate monitors now claim to be able to measure the amount of calories one expends during exercise. Since weight loss is one of the primary reasons why people exercise, the ability to track daily caloric expenditure is an important part of a weight-loss program. Exercise calories can represent a varied and significant part of one's daily expenditure, so a product that can provide a calorie number can be an important tool, as well as a powerful motivator.
Exercise equipment manufacturers realized this a number of years ago and began to include caloric expenditure as part of the equipment display. While some types of exercise movements (treadmill walking/running, cycling on an ergometer, or stair climbing) have well-established calorie prediction equations that can easily be programmed into an equipment display, others, like elliptical cross trainers, do not. Some years ago, it was revealed that many manufacturers were significantly overestimating calorie expenditure on their machines, in an effort to attract more users. So while some machines from major manufacturers like Life Fitness and Precor provide pretty reliable numbers, the public has grown to view all machine readouts with suspicion.
Unfortunately, that same level of skepticism is often tossed aside when considering the calorie numbers given by HRMs. Not only do many people place unquestioned faith in the numbers from their HRMs, they are using HRMs to measure calories for unrelated activities, such as strength training and yoga, and are wearing HRMs for 24 hours to record daily caloric expenditure.
We need to look at how HRMs estimate calories and under what conditions are these numbers valid.
First of all: Heart rate monitors do not measure anything except heart rate. They do not measure oxygen uptake or caloric expenditure directly--just heart rate.
Caloric expenditure is actually related to oxygen uptake (VO2). When the body produces energy, it consumes oxygen. As work intensity increases, oxygen uptake increases. By measuring oxygen uptake, we can measure caloric expenditure. During dynamic, aerobic exercise, the increased need for oxygen is met via increased cardiac output. At fairly low levels of exertion, the heart reaches maximum stroke volume (the amount of blood pumped by each heart beat), therefore cardiac output can only be increased by increasing heart rate.
During aerobic exercise, there is a consistent relationship between heart rate and oxygen uptake. If heart rate increases by X%, oxygen uptake increases by essentially the same percentage. If we know one's percentage of HRmax during exercise, we can estimate their percentage of VO2 max and use that number to estimate oxygen uptake, and thus caloric consumption. To determine that estimated number, the HRM must be programmed with the user's resting heart rate (HRrest), maximum heart rate (HRmax), maximal oxygen uptake (VO2max), age, gender, and weight.
In summary: if we know your VO2 max is 40 and we know that you are working at 70% of HRmax and we know that 70% of HRmax is equivalent to 57% of VO2 max, we can estimate that your oxygen uptake is 57% of 40 (or 22.8) and we can easily estimate caloric expenditure from that number. (HRMs account for the continuous fluctuations that take place during exercise, so this is not math that you can realistically do on your own workouts).
Now this is an oversimplification, but that's the basic concept. In reality, there are a number of other variables to take into consideration and Polar uses much more sophisticated algorithms to make their calculations, but the basic principle is the same.
Because the caloric expenditure is inferred rather than directly measured, there are several points in the process where they can be in error.
First the main point: heart rate can only be used to estimate caloric expenditure when the heart rate/oxygen uptake relationship exists--i.e. during aerobic exercise.
If heart rate increases without a corresponding increase in cardiac output, oxygen uptake (and thus caloric expenditure) does NOT increase.
Weight lifting, thermal stress, illness, dehydration, emotional stress, proportionately greater amount of arm work, and cardiovascular drift during aerobic exercise--these are all conditions under which an increase in heart rate will NOT be matched by an increase in caloric expenditure to the same degree. Under these conditions, HRMs may significantly overestimate caloric expenditure.
Cardiovascular drift refers to a condition where, during prolonged aerobic exercise, heart rate increases without any increase in workload. This is thought to be due primarily to loss of fluid and increased body temperature. During a 45-60 min workout, heart rates in some individuals can increase 10-20 beats/min when maintaining the same workload. I have observed that during a 45 min stairmaster workout, my Polar calorie reading for the second half of the workout will increase by 25% over the first half, even when workload is kept constant.
(Disclosure: I do not know what programming Polar has done, if any, to address cardiovascular drift. I have not found any information on the net one way or the other and the Polar reps I have asked about this didn't know the answer. I am basing my conclusions on my experience and those of others I have worked with).
So you should not rely on calorie readings that are recorded during traditional strength training (e.g. lifting weights with a standard set/reps routine), at rest, or during activities in which your heart rate is less than 100 beats/min.
Does this mean HRMs are not that reliable? Not at all. The points of potential error I have identified are all due the complexity and variability of human physiology, not any shortcomings by Polar or others. Quite frankly, it is impressive to me that they can be as accurate as they are.
And for activities that involve intermittent, varied activities, or unmeasured workloads--e.g. aerobic classes, spinning classes--an HRM is the only reliable measuring tool you have.
My primary motivation for presenting this information is to give people information to make informed choices and to dispel the ideas that HRMs directly measure calories and represent a "gold standard" for estimating calories expended during exercise.
More research is being done to improve the accuracy of the caloric estimates. A company called Firstbeat Technologies claims that they have analyzed minute fluctuations in heart rate intervals that allows them to not only estimate calorie expenditure with improved precision during many activities, but also estimate post-exercise oxygen consumption (EPOC) and degree of recovery from a previous workout. They have licensed their technology to Suunto and it is included in their Tc series of watches (only the top model includes the EPOC function). I am sure Polar is doing similar research--both companies are headquartered in Finland.
How to get the most out of your HRM?
Your HRM will only be as good as the information you enter in the basic setup. For best accuracy, you need to enter the most accurate data possible during the setup and update that information regularly as necessary.
Age, Gender, Height, Weight: These are pretty easy and self-explanatory. Just remember to update your weight when it changes by a notable amount (say 5 lbs).
Resting Heart Rate: Again, this should be pretty easy to measure. Check it first thing in the morning before getting out of bed; do it 2 or 3 days to get a consistent number. If you are starting a program, or recommitting to a program, this may also change over time, so recheck periodically and adjust as necessary.
Maximum Heart Rate: This is one of the tricky ones, but also a crucial one to get as accurate as possible. You can start with one of the various age-prediction formulae (e.g. 220-age; there are a bunch on the net--google "maximum heart rate calculator" and take your pick).
Keep in mind that a number of people have actual maximum heart rates that are significantly higher than "average"--your HRM will probably calculate your target HR range for you. Everyone should always compare recommended target HR with your perceived exertion. If your heart rate seems really high, but you don't feel you are working that hard, you will have to go into setup and increase that max HR number. Absent a max exercise test, you will just have to do some trial and error.
Maximum Oxygen Consumption: This is even trickier, because, unlike HRmax, many people do not have a clue where to even start on this number. Polar has a "fitness test" that uses resting heart rate to estimate VO2 max. I am very skeptical, but I have never been able to try it out on myself--either my rest HR is too low or when I have tried it, it was too irregular, but I have never gotten anything but error readings. I would start with the Polar number and go from there. Some machines have a submax test protocol available as a program--you could try one of those, or some facilities offer a fitness assessment that includes a submax test--if you have had one of those recently, you could use that number as well.
Hopefully, this will help you get the most out of your heart rate monitor.
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